1. Field of the Invention
The present invention relates to an apparatus for even distribution of dry hard particles to the surface of a continuously fed paper web, a process for providing the surface of a decor paper or an overlay paper for an abrasion resistant laminate with an even layer of small, hard particles and a particle coated decor paper or overlay paper produced by the process.
Products covered with a decorative thermosetting laminate are frequently used today. They are mostly used where the demands for abrasion resistance are high, but also where a resistance against different chemicals and humidity is required. Floor boards, floor skirtings, table tops and wall panels can be mentioned as examples on such products.
2. Description of the Related Art
Decorative thermosetting laminates are often made of two to seven Kraft paper sheets impregnated with phenol-formaldehyde resin and a decor paper sheet impregnated with melamine-formaldehyde resin or another thermosetting resin. The decor paper sheet can be monochromatic or patterned for instance with a wood pattern or a fancy pattern and placed as a top layer in the laminate.
Often one or more so-called overlay sheets of xcex1-cellulose usually impregnated with melamine-formaldehyde resin are placed on top of the decor paper to protect the decor paper sheet from abrasion.
There are also laminates consisting of a base layer of particle board or fibre board provided with such a decor paper sheet and possibly an overlay sheet. These sheets can be laminated towards the base layer under heat and pressure. If a decor paper only is used and no overlay sheet, the decor paper sheet can be glued towards the base layer instead.
To further increase the abrasion resistance of the decor paper sheet and/or the possible overlay sheets these may be provided with a coating of hard particles. These particles can be applied to the paper by mixing them into the thermoseting resin used for impregnating the paper. The particles can also be added to the wet cellulose fibers on the wire of a paper machine. Finally it is known to coat the resin impregnated paper with hard particles by applying the hard particles onto the paper before drying the resin.
The first method is illustrated for instance in U.S. Pat. No. 4,473,613. This method results in an uneven distribution of the hard particles and thereby an uneven abrasion resistance of the decorative laminate. The reason is that it is very difficult to disperse average size and bigger particles in a resin solution since these due to their higher density will sink to the bottom of the vessel used for storing the resin. Such a dispersion will therefore be practically unusable since the number of hard particles per surface unit will vary as time goes. This problem can partly be counteracted by increasing the viscosity of the resin solution by an addition of a thickener. However, such additives will deteriorate the properties of the resin and give a worse end result. In addition, even with a thickener it will be difficult to change the amount of hard particles per surface unit if so desired since also the resin content will be changed.
The second method mentioned above can be illustrated by the U.S. Pat. No. 3,798,111. The method disclosed in this patent is generally used for the production of overlay paper of xcex1-cellulose. The hard particles for instance of aluminium oxide are then spread over a layer of wet xcex1-cellulose fibers on the wire of a paper machine. With this method, the hard particles are distributed more or less irregularity within the whole fibre layer. Some of the particles even pass through the wire and cause serious pollution problems in the paper making machine. In the overlay paper obtained the hard particles will be distributed in an uncontrollable way. It is impossible to get an even distribution of the hard particles on the surface of the paper, where they give the best effect against abrasion.
In the above U.S. Pat. No. 3,798,111 a decor paper is made with the method disclosed, whereupon a decor is printed on top of the produced paper. Since the hard particles are situated below the decor they cannot possibly give an increased abrasion resistance. In spite of the disadvantages mentioned the method is videly used commercially for the production of abrasion resistant overlay sheets.
The third method mentioned above can be illustrated by our own U.S. Pat. No. 4,940,503, where the hard particles are applied to a continuous decor paper or an overlay paper which is impregnated with a liquid solution of a thermosetting resin. The resin is wet when the particles are coated on the paper. The paper is dried when the particles have been added.
The particles are distributed by means of a device comprising a container containing the hard particles and a rotating doctor-roll with an uneven surface placed under the container, whereby the particles are intended to fall from the container to the doctor-roll and then be evenly distributed on the paper web fed under the doctor-roll. The device usually contains an air knife intended to get the particles to come loose from the doctor-roll at a constant amount per unit of time.
Decorative thermosetting laminates produced for flooring boards where at least one overlay has been provided with hard particles by said method have been tremendeously successful.
The method is by far the best commercial one for production of highly abrasion resistant decorative thermosetting laminates. The particles are distributed very evenly on the paper web.
However, sometimes you find clusters of particles sticking together on the surface of the coated paper resulting in patchy or hazy areas. Between these clusters there are minor areas lacking particles. If the formation of such clusters could be avoided by an even better distribution of the particles the abrasion resistance would increase without addition of a higher amount of hard particles. A decrease of the cluster formations would also improve the decorative effect of the decorative laminate. Thus, there is a need for improvement of this process for even distribution of hard particles to the surface of a continuously fed paper, especially an overlay paper for abrasion resistant laminates. These laminates constitute the top layer of flooring boards which usually have a base layer of particle board or fibre board to which the laminate is glued. The flooring boards are furnished with groove and tenons in the side edges as ordinary flooring board of wood.
According to the present invention it has been possible after an extensive development work lasting for years to meet the above need. Thus, the present invention relates to an apparatus for even distribution of small hard particles to the surface of a continuously fed paper web impregnated with a liquid thermosetting resin composition, the resin being wet at the distribution of the hard particles. The apparatus includes a feed hopper containing the hard particles. The hopper has an outlet extending transversely of said fed paper web. A rotating doctor-roll preferably with an uneven surface is placed under the feed hopper and is in communication with said outlet for reception of hard particles therefrom. The doctor-roll is in spaced substantially parallel relation to said paper web fed under the doctor-roll. The apparatus also has a means for releasing the hard particles from the doctor-roll and distributing them evenly on the fed paper web. This means comprises an electrode arrangement placed between the feed hopper and the downwards directed vertical tangent (T) of the doctor-roll. The electrode arrangement is preferably enclosed by a casing provided with a downwards directed sliding plate, whereby the hard particles are lifted from the doctor-roll and fluidized by means of an electric field between the electrode arrangement and the doctor-roll resulting in an even amount of particles falling down on the paper web fed under the doctor-roll.
As mentioned the surface of the doctor-roll is preferably uneven. Suitably this unevenness consists of 1-100 xcexcm, preferably 30-70 xcexcm deep grooves oriented axially, radially or diagonally over the surface of the doctor-roll. The unevennesses can also consist of pits with the same depth as above. However, it might also be possible to use a doctor-roll with an even or rather even surface.
The feed hopper is suitably provided with a scraper plate at its outlet to give an even feeding of the particles along the surface of the doctor-roll. The scraper plate will allow the particles situated in these grooves or pits to be brought in the rotation of the doctor-roll while the other particles are prevented from coming with the doctor-roll at the rotation thereof. In this way the amount of particles can easily be controlled by amendment of the rotation velocity of the doctor-roll as the doctor-roll will always be fed with a fixed amount of particles per surface unit from the feed hopper. The amount of particles fed per surface unit depends on the depth of the grooves or pits, on the distance between these grooves or pits and also on the particle size.
Preferably the doctor-roll, the feed hopper and the paper have about the same voltage potential while the electrode arrangement has a positive or preferably a negative voltage potential of at least 1 kV as compared to the doctor-roll. The doctor-roll is preferably grounded and thereby uncharged. At least the surface of the doctor-roll is made of a conducting material, suitably a metal. The suitable voltage potential depends on the distance between the electrode arrangement and the doctor-roll, the particulated material and to a certain extent the moisture of air. By changing the distance and the voltage potential also the intensity of field will be changed. However, intensities of field which can result in lead should be avoided.
Voltages amounting from 1-15 kV might be useful but tests have shown that 2-8 kV is enough at a distance between the doctor-roll and the electrode of 5-20 mm with aluminium oxide particles with an average size of 40-90 xcexcm. Suitably the aluminium oxide particles have the form xcex1-Al2O3 which is not hygroscopic. The distance between the doctor-roll and the electrode may be about 2-50 mm, preferably about 3-30 mm or 5-20 mm.
The sliding plate mentioned above is used to guide the stream of particles to the paper. Preferably the sliding plate is placed in such a way that at least its extension will cross the downwards directed tangent (T) at an angle of 5-50xc2x0. The sliding plate is possibly followed by further sliding plates which are placed under the first sliding plate and have an angle towards the vertical tangent (T) of 5-50xc2x0. As to the angle of the sliding plates towards the vertical tangent (T), small angles such as angles within the interval 5-25xc2x0 are preferred since differences in friction coefficient between the particles can influence their sliding velocity. The difference in sliding velocity caused by the difference in friction coefficient will increase at larger angles. This can have an impact on the dispersion degree of the particles on the paper surface.
The sliding plates are made of an conducting material, suitably a metal. Preferably, the sliding plate and possible further sliding plates have the same voltage potential as the doctor-roll. Since the sliding plates are grounded and thereby mainly lack deficit or surplus of electrones any possibly remaining charge of the hard particles will be conducted away by the sliding plates. Thereby the particles will not be attracted to the sliding plates and stick thereto. In addition the particles are completely uncharged when they reach the paper.
The lower edge of the lowermost sliding plate is preferably situated only a millimetre up to a few centimetres from the paper surface. If only one sliding plate is used this will of course be regarded as the lowermost one. Thereby, the sliding plate will in addition to conducting away possible charges from the particles also give a sharply limited distribution zone. This will decrease the disturbing influence of air streams and thereby give a more even distribution of the particles on the paper surface.
In order to prevent the hard particles from sticking to the sliding plate and possible further sliding plates, for instance due to surface tension or other similar phenomena, the plates are suitably arranged in a resonance free vibration by means of a vibrator. In this way the hard particles are prevented from stopping or sticking to the sliding plate or the further sliding plates at their passage towards the paper. The vibrator can either work directly with the plates or be of an acoustic type and thereby indirectly bring the sliding plates and the surrounding air in vibration.
Suitably the surface of the sliding plates facing the distributed hard particles is polished to a highly glossy state. In this way the particles will also be prevented from sticking to the sliding plates.
Since the hard particles normally are very small the coating process can be disturbed by air streams which can cause local clusters of particles and local areas without particles. Therefore, the device is suitably encased and possibly provided with aerodynamic spoilers near the paper for avoiding disturbing air streams.
The electrode arrangement suitably consists of one or more electrodes preferably of a semiconducting material such as phenolic resin. The electrode/electrodes are electrically connected to a voltage source via an electrically conductive distributor which suitably runs along the main part of the horizontal extension of the electrode/electrodes. Alternatively the electrode/electrodes can consist of a conductive material, but in this case said material must be coated with an insulating material. The longitudinal side edges and all corners of the electrode/electrodes should have a rounded surface, since otherwise the electric field will be concentrated there which might result in a formation of clusters of particles.
Preferably the electrode arrangement is adapted to and at least mainly corresponds to the width of the doctor-roll. The electrode arrangement is suitably placed parallelly to the axis of the doctor-roll.
The hard particles are preferably made of a conducting material, for example aluminium oxide or a semiconducting material for example silicon carbide or silica. If semiconducting particles are used suitably the moisture of air in the space between the electrodes and the doctor-roll is increased. In this way the ability of the semiconducting particles to take charge and thereby be polarized is increased. The particles suitably have an average size of about 20-150 xcexcm, preferably 40-90 xcexcm. Usually the particles are dry, but sometimes they can contain a certain amount of liquid, preferably water. However, the liquid content should not be so high that the particles are agglomerating.
The above apparatus according to the present invention with the perfectly working electrode arrangement for releasing the hard particles from the doctor-roll instead of a previously known air knife gives an outstanding evenness of hard particles on the surface of the coated paper web.
The present invention also relates to a process for providing the surface of a decor paper or an overlay paper for an abrasion resistant laminate with an even layer of small hard particles, said process comprising impregnating a continuously fed web of said paper with a liquid thermosetting resin composition and having the surface of the paper wetted with said resin, coating at least one side of the paper web with 2-20 g/m2, preferably 3-15 g/m2 of small and hard particles so that the particles are evenly distributed over the surface of resin on the paper web. Then the resin with the particles coated thereon is dried. The small and hard particles are applied by means of an apparatus including a feed hopper containing the hard particles, the hopper having an outlet extending transversely of said fed paper web, a rotating doctor-roll preferably with an uneven surface placed under the feed hopper and being in communication with said outlet for reception of hard particles therefrom, said doctor-roll being in spaced substantially parallel relation to said paper web fed under the doctor-roll, and a means for releasing the hard particles from the doctor-roll and distributing them evenly on the fed paper web. The process is characterized in that the particles are released by a means comprising an electrode arrangement placed between the feed hopper and the downwards directed vertical tangent (T) of the doctor-roll, the electrod arrangement preferably being enclosed by a casing provided with a downwards directed sliding plate, whereby the hard particles are lifted from the doctor-roll and fluidized by means of an electric field between the electrode arrangement and the doctor-roll resulting in an even amount of particles falling down on the paper web continuously fed under the doctor-roll.
The thermosetting resin used according to the present invention is preferably selected from melamine-formaldehyde resin and radiation curing resins such as epoxy acrylate oligomer, polyester acrylate oligomer, urethan acrylate oligomer, methacrylate oligomer, silicon acrylate oligomer and melamine acrylate oligomer. The radiation curing comprises electron beam curing and UV curing. Usually the thermosetting resin is present as an aqueous solution.
According to one embodiment of the invention one side of the paper is provided with hard particles with an average size of about 40-150 xcexcm, preferably 40-90 xcexcm by the disclosed method. The other side of the paper may then be impregnated with the above thermosetting resin containing above hard particles but with a size of 1-30 xcexcm, preferably 1-10 xcexcm. This coating preferably gives an addition of hard particles of 1-20 xcexcm . Alternatively the two impregnating steps may be made on the same side of the paper with an intermediate drying step.
The present invention also relates to a particle coated decor paper and/or overlay paper produced by the above process.
At the production of a decorative thermosetting laminate one or more particle coated overlay papers can be used together with one or more decor papers with or without any hard particles.